The present disclosure relates to a battery pack applied to, e.g., a lithium ion secondary battery.
In recent years, various types of portable electronic devices, such as videotape recorders (VTRs) with camera, cell phones, and laptop computers, are widely used, and those having smaller size and weight are being developed. As the portable electronic devices are miniaturized, demand for battery as a power source of them is rapidly increasing, and, for reducing the size and weight of the device, a battery for the device must be designed so that the battery is lightweight and thin and the space in the device can be efficiently used. As a battery that can meet such demands, a lithium ion secondary battery having a large energy density and a large power density is preferred.
Especially, batteries having high selectivity of shape, sheet-type batteries having a reduced thickness and a large area, or card-type batteries having a reduced thickness and a small area are desired. For meeting the demands, as described in TAKAMI Norio, “Ultra thin lithium-ion battery using aluminum laminated film case”, Toshiba Review, Toshiba Corporation, Vol. 56 No. 2, February 2001, pp. 10-13, a thin battery is produced using an external packaging material in a film form, such as an aluminum laminated film, making it possible to obtain the above-mentioned battery having a reduced thickness.
FIG. 1 shows the appearance of a battery 1 described in TAKAMI Norio, “Ultra thin lithium-ion battery using aluminum laminated film case”, Toshiba Review, Toshiba Corporation, Vol. 56 No. 2, February, 2001, pp. 10-13. The thin battery is produced by covering a flat-type battery element, which is formed by stacking a positive electrode and a negative electrode through a separator and spirally winding them, with an aluminum laminated film, and sealing the aluminum laminated film around the battery element. A positive electrode terminal 2a and a negative electrode terminal 2b (hereinafter, frequently referred to as “electrode terminals” unless otherwise specified) connected to the positive electrode and negative electrode are electrically introduced from the battery, for example, from one side of the thin battery 1, and the aluminum laminated film around the battery element except for one side is sealed and then an electrolytic solution is charged from the unsealed opening, and finally the side of the film from which the electrode terminals are electrically introduced is sealed, thus obtaining the above-mentioned thin battery.
The battery of this type uses an aluminum laminated film having a thickness of about 100 μm as external packaging, and hence has small battery strength, as compared to a battery using a metallic can, and it is difficult to use this battery as a battery pack as it is. Therefore, a battery pack containing a battery element covered with a laminated film in a pack housing made of a plastic is widely used.
However, the battery pack using the above pack housing has a small metal ratio in the external packaging portion and hence has poor heat radiation properties. Therefore, when accidental heat generation occurs, the temperature in the battery pack tends to increase.
For solving the problem, Japanese Patent Application Publication No. 2003-303580 has proposed a battery pack 10 shown in FIG. 2, which includes a battery element 12 covered with a laminated film and contained in an external packaging case 11 made of a metal having satisfactory strength and having an opening at one end, and a circuit board 13 attached to the battery element. JP2003-303580 has a description showing that, by using aluminum having a thickness of about 150 to 200 μm or the like as the external packaging case, a battery pack having high strength can be obtained.
Japanese Patent Application Publication No. 2004-165134 discloses a battery pack 20 having a construction shown in FIG. 3, which includes a battery element 21 covered with a wrapping material, a circuit board 24, a frame 22 provided around the battery element 21, and a wrapping material 23, having a strength higher than that of the wrapping material for covering the battery element 21, for covering the battery element 21, the circuit board 24, and the frame 22. The frame 22 used in the battery pack in the patent document 2 can be formed from a plastic material, and therefore the frame not only can secure the battery strength, but also can protect the battery element from an impact, such as a drop impact.
Further, Japanese Patent Application Publication No. 2005-166650 includes a description of a battery pack using a rigid laminated film in part of an external packaging material, wherein the rigid laminated film constitutes the outermost layer of the battery.
FIGS. 4A to 4C and FIGS. 5A and 5B are diagrammatic views showing constructions of the battery in JP 2005-166650, which is being fabricated. In JP 2005-166650, a flexible laminated film 31 having a recess portion 31 a and a rigid laminated film 32 are used, and a battery element is contained in the recess portion 31a. The rigid laminated film 32 is then placed over the flexible laminated film 31 so that it covers the opening of the recess portion 31a, and the flexible laminated film 31 and the rigid laminated film 32 are sealed at the stacked portion around the battery element. Subsequently, as shown in FIG. 5B, the rigid laminated film 32 and flexible laminated film 31 are molded so that they contain a battery element 35, and a circuit board and resin molded cover are provided, thus producing a battery pack 30 in which the rigid laminated film 32 constitutes the outermost layer. This battery pack has excellent volume efficiency and achieves a high battery strength.
However, the above described related art battery packs have the following problems. Specifically, in the battery pack having the construction described in JP 2003-303580, the battery element covered with a laminated film is further placed in the external packaging case having a thickness of about 150 to 200 μm, and hence high battery strength can be achieved, but the volume efficiency is low.
In the battery pack having the construction described in JP 2004-165134, the use of the frame achieves high battery strength, but the volume efficiency is low.
Further, in the battery pack having the construction described in JP 2005-166650, the rigid laminated film is difficult to mold, and therefore the rigid laminated film and the flexible laminated film must be used in combination to cover the battery element. In stacking the rigid laminated film and the flexible laminated film and sealing them, the four sides of the stacked films around the battery element must be individually bonded by heat seal or the like. In the construction of this battery, at the top portion on the side from which the electrode terminals of the battery are electrically introduced and at the bottom portion on the opposite side, the sealed width is inevitably smaller, and therefore moisture easily enters the battery from these portions, causing a problem in that the battery life is shortened or the battery performance becomes poor.
Accordingly, it is desired to provide a battery pack which is not only thin and has both excellent volume efficiency and predetermined battery strength, but also in that it has excellent sealing properties and excellent heat radiation properties.